A lake like Mirror Lake is highly prized for its aesthetic and recreational value. Users of lakes commonly perceive them as static features because they look pretty much the same every time they visit. However, a lake is actually very dynamic, receiving new water from precipitation, stream inflow, and ground water inflow, and losing water to evaporation, stream outflow and ground water outflow. Transported along with the water are other materials, such as chemicals dissolved in the water and particles of mineral and organic matter suspended in the water. These gains and losses of water and other materials affect the amount of water and materials that are present at any one time in the lake. Keeping track of the gains, losses, and changes of water and material stored in the lake is done by taking a budget approach; that is, losses should equal gains, plus or minus the change in storage. The concept of a budget is deceptively simple, but the ability of hydrologists to measure the inflows and outflows is difficult to do accurately, and is therefore fraught with uncertainty.

As you take the hydrology tour, the stops will have a description, and some calculations you can make yourself, of how the various components of the hydrologic system are measured. At the end of the tour, Stop 10, all of this information is brought together to result in the water budget of Mirror Lake. By the end of the hydrology tour, we hope you will have some appreciation of the complexities of the hydrologic system, and of how ever-changing are the exchanges of lake water with the various components of the hydrologic system.